1. Field of the Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of dichlorodiphenylsulfone (DCDPS) isomers. More specifically, the invention relates to a process for separating 4,4'-DCDPS mixture comprising 4,4'-DCDPS and one or more additional DCDPS isomers, which process employs an adsorbent comprising a crystalline aluminosilicate to selectively adsorb all isomers except 4,4'-DCDPS from the feed mixture and remove 4,4'-DCDPS as the product in the raffinate.
2. Description of the Prior Art
The use of crystalline aluminosilicates to perform a number of separations is well known in the separation art. Examples of such separations are the use of zeolites to separate normal paraffins from branched chain paraffins, (U.S. Pat. No. 2,985,589), faujasites to separate olefinic hydrocarbons from paraffinic hydrocarbons (U.S. Pat. No. 3,265,750), zeolites to separate specific monosaccharides or classes of monosaccharides from carbohydrate feed mixtures (U.S. Pat. No. 4,024,331), etc.
This invention is particularly concerned with the separation of 4,4'-DCDPS from other isomers of DCDPS. Obtaining pure 4,4'-DCDPS has commercial significance in light of its potential as a monomer or co-monomer for the production of polysulfone resins, polyethersulfone resins, polyarylsulfone resins, etc. A source of the isomer mixture is the sulfonation of chlorobenzene with SO.sub.3 and thionylchloride or through a series of well-known reactions or the reaction of dimethylpyrosulfate with monochlorobenzene, both of which produce a mixture of DCDPS isomers. It is desirable to use very pure 4,4'-isomer as the polymerization reactant. Typically, however, the initial purification is by crystallization, which can recover about 60% of the 4,4'-isomer, but a means must be found to economically recover 4,4'-DCDPS in concentrations greater than about 90-95%, and, ideally, greater than 98% in order to manufacture high quality polysulfone resins. Recrystallization is effective for the concentration proposed, but more costly than a chromatographic adsorptive separation.
Current methods of separating DCDPS isomers include filtration of isomer reaction mixture following precipitation with a basic aqueous solution (U.S. Pat. No. 3,309,409) and crystallization with monochlorobenzene as solvent (U.S. Pat. No. 3,334,146).
While crystalline aluminosilicates or zeolites have been used in adsorption separations of various mixtures in the form of agglomerates having high physical strength and attrition resistance, to our knowledge an effective chromatographic separation process for DCDPS isomers has not been found. Methods for forming the crystalline powders into agglomerates are also known and include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to a high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. As binders, clays of the kaolin type, water permeable organic polymers or silica are generally used.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRossett U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al. U.S. Pat. No. 4,642,397 is incorporated herein.
It has now been discovered that X- or Y-type zeolites exchanged with cations at cation exchange sites selected from Groups IA or IIA are suitable adsorbents for the separation of 4,4'-DCDPS from other isomers of DCDPS, provided certain conditions in the chromatographic separation process are maintained. Important parameters to be controlled in the process are water concentration of the adsorbent, temperature of the process and concentration of desorbent components. Moreover, by our process, it is estimated that 4,4'-DCDPS can be obtained at purities of at least 95% to as high as 99+% with recoveries at 90-99%.